1. Field of the Invention
The present invention relates to an improved mass flow controller comprising a mass flow meter portion for measuring a flow rate of a fluid and a fluid-controlling portion for controlling the flow rate of the fluid provided between a fluid inlet and a fluid outlet formed in a base member
2. Description of Related Art
In the prior art there is known a mass flow controller comprising a capillary tube having certain characteristics provided in a mass flow meter portion and at least one capillary tube having the same characteristics provided in a bypass portion. An exemplary prior art mass flow controller is disclosed in Japanese Patent Publication No. Sho 59-41126. The above-described prior art has shown the following disadvantages:
As a rule, a mass flow controller of this type has been used in a supply system of gases used for the production of semiconductors and the like in many cases. The gases to be controlled include many kinds of inert gas (such as N.sub.2 and Ar), combustible gas (such as H.sub.2, SiH.sub.4 and B.sub.2 H.sub.6), highly corrosive gas (such as HCl, Cl.sub.2, WF.sub.6 and BCl.sub.3), and the like. In order to prevent the supplied gases from being contaminated with moisture left on an inner wall of the mass flow controller during the production of semiconductors, it is necessary to bake them out by flowing inert gases, such as highly pure N.sub.2 (having the dew point of -100.degree. C. or less), through the mass flow controller as occasion demands to remove said moisture. This baking-out process is conducted by winding a tape heater around a gas piping system including the mass flow controller, or by directly electrifying said system, or by putting said system as a whole in a heat insulating vessel to heat it to an appointed temperature.
In the mass flow controller disclosed in the above-described Japanese patent, a ratio of the largest flow rate in the flow rate-controlling valve during the time when the flow rate is not being controlled to that during the time when the flow rate is being controlled amounts to about 5:1. In particular, with the mass flow controller of which the largest controlled flow rate is less than 100 cc/min, a disadvantage has occurred in that even though the baking-out process is continued for three months at a temperature of 120.degree. C., a quantity of moisture is reduced to only 20 to 30 ppb.
This will be described below in more detail as to the case where, for example, the mass flow controller is used in a process gas supply system for producing semiconductors.
A thin film, which is used for large-scale integrated circuits (LSI), formed of polysilicon, SiO.sub.2, Si.sub.3 N.sub.4, Al, Al-Si and Al-Si-Cu is formed by sputtering or CVD (Chemical Vapor Deposition).
Generally, in the film formation by sputtering, Ar is used as a gas to be flown through the sputtering device. If a pattern of LSI is minute, aspect ratios of a contact hole and a through hole connecting a silicon substrate with the top and bottom of wiring and multi-layer wiring are increased. If the thin and deep contact hole and through hole, having a diameter of submicrons, are to be neatly filled with electrode materials, the process gas pressure during the sputtering is reduced to, for example, the order of 10.sup.-3 Torr. If it is intended to hold a quantity of exhaust gas by a pump for the exhaust of the process chamber at a practical one, it is natural that a flow rate of Ar gas supplied is reduced with a reduction of process gas pressure.
On the other hand, the gases used in the film formation by CVD include SiH.sub.4, Si.sub.2 H.sub.6, WF.sub.6, NH.sub.3 and the like and, in general, a flow rate thereof is large.
In addition, with a device for RIE (Reactive Ion Etching) for etching minute patterns and a device for ion injection, the flow rate of gases is less than 100 cc/min in almost all cases.
The largest flow rate of the mass flow controller used for the precise control of such a small flow rate of gas is usually at most 100 cc/min.
The mass flow controller has been a part which is frequently clogged by reactive gas sediment and the like. This clogging trouble is not caused by the mass flow controller itself, but rather by SiO.sub.x powders and the like, which are reaction products of the moisture, which is the main ingredient of gases entering the gas piping system from outside or gases escaping through an inner wall of the gas piping system and, for example, SiH.sub.4 and Si.sub.2 H.sub.6 in almost all cases.
In addition, the technological advancement of parts and techniques, such as butt welding, has, for the last four to five years, led to a reduction in the quantity of gases entering the gas piping system to 1.times.10.sup.-11 Torr-liter/sec (limit of the detector) or less. At present, gases comprising moisture as the main ingredient discharged from the inner wall of the gas piping system is the largest source of pollution.
Accordingly, the gas piping system including the mass flow controller and the like is assembled and then purged with ultrahighly pure N.sub.2 gas or Ar gas to remove pollutant molecules adsorbed into the inner wall of the gas piping system. It is necessary that difficult to remove molecules, such as water, be baked at at least 100.degree. C. or more.
In view of the high possibility of complications and the necessity of periodical flow rate compensation, the mass flow controller is incorporated in the gas piping system in the form of a unit as shown in FIG. 2. Referring to FIG. 2, reference numeral 101 designates a mass flow controller, reference numerals 102, 103, 104, 105 designate stop valves, reference numeral 106 designates a gas flow inlet, reference numeral 107 designates a gas flow outlet, and reference numeral 108 designates a bypass line. The stop valves 102 and 104 and the stop valves 103 and 105 form an integral double three-way monoblock valve, respectively. When the process gases are flowed, the stop valves 102 and 103 are opened, while the stop valves 104 and 105 are closed. When the mass flow controller 101 is removed, the stop valves 102 and 103 are closed, the stop valves 104 and 105 are opened, and then a purge gas, such as N.sub.2 or Ar, is flowed in the course of the gas flow inlet 106.fwdarw.the bypass line 108.fwdarw.the gas flow outlet 107 to carry out the desorption.
FIG. 3 shows the gas purge effect in the case where the mass flow controller having a maximum flow rate of 20 cc/min is incorporated in the gas piping system of the device for RIE.
Referring to FIG. 3, the axis of ordinate shows the dew point and moisture concentration of N.sub.2 gas used for the purge immediately before it passes through the gas piping system to flow into the device. The axis of abscissa shows a number of days for which the purge has been continued. The quantity of moisture in N.sub.2 gas used for the purge is 1 ppb or less. The relation of the dew point and the moisture concentration has already been shown in the respective handbooks. Roughly speaking, -90.degree. C.: 95 ppb, -100.degree. C.: 14 ppb, -110.degree. C.: 1 ppb.
The thick full line in FIG. 3 shows a state in which the purge is carried out through the bypass line with N.sub.2 gas having a flow rate of 5 liters/min. The thin full line shows a state in which the purge is carried out through the mass flow controller with N.sub.2 gas having a flow rate of 100 cc/min. In addition, the upper oblique lines in FIG. 3 show that the whole system is heated to 120.degree. C. when the gases are flowing, while the portions not having oblique lines show that the whole system is not heated. It is found that if the purge is carried out with N.sub.2 gas having a flow rate of 5 liters/min, the dew point arrives at -100.degree. C. (moisture concentration of 1 ppb) after several days, but, in the case where the purge is carried out through the mass flow controller with N.sub.2 gas having a flow rate of 100 cc/min, the dew point is lowered to about -95.degree. C. and the moisture adsorbed into the inner wall of the mass flow controller cannot be completely removed. In addition, the purge was carried out through the mass flow controller with N.sub.2 gas for three months with repeated baking, but the dew point reached at most -97.degree. C. (moisture concentration of 25 ppb).
It is absolutely required for the enhancement of the performance of the process that the purge of the mass flow controller lead to the dew point of -110.degree. C. (moisture concentration of 1 ppb) for at least about one week.
The present invention has been achieved paying attention to the above-described matters.